The present invention relates to epoxy functional polyester resins, having an increased molecular weight, to a process for their preparation and to outdoor durable powder coating compositions comprising them.
Triglycidylesters which can be used in good quality outdoor durable coatings and in moulding compositions are disclosed in European Patent Application No. 447360A (EP-A-447,360). Due to the anhydride half ester nature of the terminal carboxyl functions present in the tricarboxylic acid adduct precursors, strong alkaline conditions should be avoided during glycidation of these tricarboxylic acid adducts to avoid hydrolysis of the glycidylester formed and/or hydrolysis of one or more ester groups in the resin backbone. As a result thereof the triglycidylester produced will contain a relatively high level of hydrolizable chlorine and/or will contain low molecular weight hydrolysis products which might cause toxicity problems, as may be derived from xe2x80x9cWater based coatings with excellent saponification stability XIIIth Int. Conf. 1987, Athens, Greece, p. 175.xe2x80x9d
The high level of hydrolizable chlorine is reflected in example 2 of EP-A-447,360 which relates to the glycidation of the 2:1 adduct of hexahydrophthalic anhydride and dimethylolpropionic acid. The product obtained has a chlorine content of 1.5%. Such a high level of residual chlorine is generally undesirable in coating compositions. In addition, due to the fact that the triglycidylesters reported in EP-A-447,360 are liquid, they can not be applied in powder coating compositions.
In International Application WO 96/11238, it was taught to a person skilled in the art of this specific area of curable coatings, that epoxy resins containing cycloaliphatic nuclei had the disadvantage that they could only provide brittle coating films when cured. This brittleness made them unsuitable for coating applications, as brittleness often led to poor adhesion.
European patent application No. 0634434A2 discloses a process for the preparation of linear tertiary aliphatic carboxyl functional polyester resins, by reacting:
(a) at least one compound Axe2x80x2 comprising one mono-functional primary- or secondary hydroxyl group and/or at least one compound Axe2x80x3 comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group;
(b) at least one aromatic or cycloaliphatic dicarboxylic acid compound B comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof;
(c) at least one diol compound C comprising two aliphatic hydroxyl groups, which may independently be a primary or a secondary hydroxyl group; and
(d) at least one dihydroxymonocarboxylic acid compound D comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl,
the molar ratio of compounds Axe2x80x2:Axe2x80x3:B:C:D being
M:N:X+Y+1:X:Y
wherein M+N=2, X ranges from 2 to 8 and Y ranges from 2-N to 8, at a temperature of from 100 to 240xc2x0 C., until essentially all the non-tertiary carboxyl groups as initially present in the reaction mixture have been reacted.
Moreover in this application were disclosed poly-glycidylester resins obtainable by reacting said linear tertiary aliphatic carboxyl functional polyesters with an excess epihalohydrin in the presence of a suitable base and optional catalyst. Preferably, the polyesters were reacted with epichlorohydrin. Both the specified linear polyesters and the corresponding polyglycidylesters derived therefrom were used with a cross-linking agent for powder coating compositions.
In European patent application No. 0720997A2, linear tertiary carboxyl functional polyesters and epoxy functional polyester resins are disclosed where these polyester resins were produced by reacting:
a) at least one aromatic and/or cycloaliphatic carboxylic acid compound A comprising two aromatic- and/or secondary aliphatic carboxyl groups or the anhydride thereof,
b) at least one hydroxyl compound B comprising two aliphatic hydroxyl groups, which groups each independently may be primary or secondary hydroxyl groups,
c) at least one hydroxyl substitute carboxylic acid compound C comprising at least one tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which groups each independently may be primary or secondary hydroxyl groups, and
d) optionally one carboxylic acid compound D comprising one carboxyl group,
the molar ratio of compounds A:B:C:D being
(X+Yxe2x88x921):X:Y:Z,
wherein X ranges from 2 to 8, Y ranges from 2 to 8, and Z ranges from 0 to 2.
These polyester resins could be used together with a suitable curing agent for the production of powder coatings, or could be converted into the corresponding glycidylesters, which in combination with a suitable curing agent could be used for the production of powder coatings.
Although the linear tertiary aliphatic carboxyl functional polyester resins and the polyglycidylesters hereof enabled a certain progress as to the requirements of excellent outdoor durability (UV stability) and resistance against hydrolysis in the cured state, with reference to their use in modern economically applied powder coatings, there is still a need for further improvement of this combination of properties.
On the other hand novel powder coating binders for the exterior durable powder coating market derived from carboxylated polyester resins, cured with epoxy functional acrylate polymers, have been proposed during the Waterborne, Higher Solids and Powder Coatings Symposium, Feb. 5-7, 1997, New Orleans La., USA, T Agawa and E D Dumain, p. 342-353, xe2x80x9cNew Two-component Powder Coating Binders: Polyester acrylate hybrid as TGIC Cure Alternative.
However, as indicated on page 353, further improvements have to be made to provide smoother films, lower cure temperatures and UV durability to rival that of automotive topcoating or outdoor building panel topcoating.
The epoxy functional polyester resins obtainable according to the hereinbefore discussed documents, although showing attractive combinations coating properties, such as outdoor durability, flexibility, hardness, chemical resistance, could not meet the present storage stability requirement from the coating industry to powder coating compositions, comprising said epoxy functional polyester resins, to enable a conveniently handling, i.e. non-blocking or caking of the powder coatings when stored at temperatures up to 40xc2x0 C.
In connection with said storage stability, the epoxy functional polyester resins aimed at, have to show an increased Tg (i.e.  greater than 40xc2x0 C.). One of the possibilities of increasing the Tg of an epoxy functional polyester resins is to increase the molecular weight (MW) or the epoxy equivalent weight (WPE).
However, the intermediate carboxyl functional polyesters to be initially prepared as starting materials for the direct glycidation process for the preparation of the corresponding epoxy functional polyester resins having the increased MW, will show a viscosity under the preparation process conditions which is unacceptable high (e.g.  greater than 80 poise at 200xc2x0 C.).
Moreover, epoxy functional polyester resins which might have been obtained by a direct glycidation of a carboxyfunctional polyester resins, which might show an even slightly increased MW, should inevitably have such a high viscosity, that the further finishing, i.e. removal of used solvents and low molecular weight by-products (devolatilization), will be impossible.
Therefore it is an object of the present invention to provide a process for the preparation of linear, epoxy functional polyesters, which show an acceptable storage stability at temperatures up to 40xc2x0 C. and which therefore show an increased Tg (over 40xc2x0 C.).
It is another object of the present invention to provide powder coating compositions, which comprise said epoxy functional polyesters aimed at, and which combine the attractive coating properties with an acceptable storage stability.
Accordingly epoxy functional polyester resins (I) are provided which are storage stable at temperatures up to 40xc2x0 C. and have a Tg of more than 40xc2x0 C. and preferably more than 43xc2x0 C. and more preferably more than 45xc2x0 C., and which are obtainable by a process comprising the reaction of an epoxy functional polyester resin (II), produced by glycidating (reaction of epihalohydrin, preferably epichlorohydrin, in the presence of a base) a carboxyl functional polyester resins (III), obtainable by reacting:
(a) at least one aromatic or cycloaliphatic dicarboxylic acid compound A comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof;
(b) at least one diol compound B comprising two aliphatic hydroxyl groups, which may independently be a primary or a secondary hydroxyl group; and optionally
(c) compound C1 comprising one monofunctional primary- or secondary hydroxyl group and/or at least one compound C2 comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group; and optionally
(d) a dihydroxymonocarboxylic acid compound D comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl; and optionally
(e) a trihydroxyalkane (E1) or tetrahydroxyalkane (E2),
the molar ratio of compounds A:B:C1:C2:D:E1:E2 being
X+Y+1:X:M:N:Y:Z:Q
wherein M+N is in the range of from 0 to 2, X ranges from 2 to 8 and Y ranges from 0 to 8, Z ranges from 0 to 2 and Q ranges from 0 to 2 at a temperature of from 100 to 220xc2x0 C., until essentially all the non-tertiary carboxyl groups as initially present in the reaction mixture have been reacted, having a WPE in the range of from 250 to 800, with a carboxy functional polyester resin derived from the herein before mentioned component (a) and (b) or with a component (a) alone, in the presence of a catalyst.
Said epoxy functional polyester resins can be suitably used for the preparation of outdoor durable powder coating compositions, showing also a sufficient storage stability in combination with attractive coating properties.
As indicated hereinbefore, the invention relates to a process for the advancement of relatively low molecular weight epoxy functional polyester resins into (WPE from 250 to 850) relatively high molecular weight epoxy functional polyester resins (WPE from 750 to 1200) by reacting the starting polyester with at least one of the diacid constituents (a) or a precondensed carboxyfunctional polyester oligomer, derived from diacid and diol constituents (a and b).
According to one preferred embodiment of the process of the present invention, an initially prepared starting epoxy functional polyester resin, having a WPE of from 250 to 500, will be converted into a similar epoxy functional polyester resin having a WPE from 800 to 1200.
For carrying this advancement reaction the epoxy functional polyester component and the diacid or polyester oligomer thereof, can be dry blended and heated to a temperature up to 180xc2x0 C. kept for a short time on said temperature and rapidly cooled or the polyester oligomer or diacid coreactant can be mixed into the premolten epoxy functional polyester to be advanced, whereafter the actual (internal) reaction temperate is kept on from 140 to 150xc2x0 C. during a reaction time of from 0.25 to 2 hours.
It will be appreciated that linear or branched final advanced resin can be produced, dependent on the type of the starting epoxy functional polyester resin.
According to a more preferred embodiment the starting epoxy functional polyester resins to be advanced according to the present invention, have been derived from carboxyl functional polyester resins produced by reacting:
a) at least one compound of the formula 
xe2x80x83wherein a xe2x89xa71 wherein R1 and R2 each may represent an alkyl group having from 1 to 4 carbon atoms, or wherein R1 and R2 may form together with the group 
a cycloalkyl group, which preferably represents 1,4-cyclohexane dicarboxylic acid (A1), optionally mixed with minor amounts of a corresponding compound of formula V, wherein a=0 or anhydride thereof (A2),
b) at least one diol compound B comprising two aliphatic hydroxyl groups which may each independently be a primary or a secondary hydroxyl group;
c) optionally a dihydroxymonocarboxylic acid compound C, comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl; and
(d) optionally a trihydroxyalkane (E1) or tetrahydroxyalkane (E2),
the molar ratio of compounds (A1+A2):B:C:E1:E2 being X+Y+2Z+3Q+P:X:Y:Z:Q, wherein X ranges from 1 to 8, Y ranges from 0 to 8, Z ranges from 0 to 1 and Q ranges from 0 to 1 and wherein P ranges from 1 to 5, and preferably 1-3 and is most preferably equal to 1, at a temperature of from 100 to 220xc2x0 C., and preferably from 180 to 210xc2x0 C. if any compound C is present, until essentially all the hydroxyl groups as initially present in the reaction mixture have been reacted.
With the term xe2x80x9cminor amountsxe2x80x9d as used the optional component A2 are meant amounts of from 0 to 10 mole %, relative to the total molar amount of A1 and A2.
Preferably carboxyl functional polyester resins are aimed at wherein Y greater than 0 if Z+Q=0, or wherein Z+Q greater than 0 if Y=0.
The process for preparation of the starting carboxyl functional polyesters, from which the initial epoxy functional polyesters to be advanced subsequently, can in general be carried out according to conventional esterification methods and preferably by azeotropic condensation, taking care that the terminal secondary carboxyl groups are only originating from 1,4-cyclohexane dicarboxylic acid. In particular, the condensation is carried out by charging the compounds A, B, optionally C and optionally D1 or D2, simultaneously to the reactor whereafter the temperature is increased from room temperature to a temperature in the range of from 180 to 220xc2x0 C., preferably from 180 to 210xc2x0 C. in the presence of any compound B, during a period of 3 to 8 hours, thus allowing the reaction to initiate and to proceed under continuous azeotropic removal of water. Generally the azeotropic removal of water is being continued until at least 90% of the original hydroxyl groups have reached and more preferably at least 95% of the original hydroxyl groups have reacted. An esterification catalyst known in the art, such as for example dibutyltinoxide, paratoluenesulphonic acid, tinoctoate, zincoctoate and lithium ricinoleate may be used in the esterification process, but is in general not required.
In order to be sure that the terminal secondary carboxyl groups have originated from the structure of formula V dicarboxylic acid wherein a  greater than 1, and in particular 1,4-cyclohexane dicarboxylic acid, and not from the corresponding 1,2-structure (a=0) and in particular 1,2-dicyclohexane dicarboxylic acid, a part of the total batch of e.g. 1,4-cyclohexane dicarboxylic acid to be included, may be added during the reaction and more preferably in its last stage.
Suitable compounds B for use in the preparation of the starting carboxy functional polyesters include branched aliphatic-, cycloaliphatic-, or araliphatic compounds, containing two aliphatic hydroxyl groups, each individually being either a primary or a secondary hydroxyl group, such as for example propylene glycol, neopentyl glycol, hydrogenated diphenylolpropane (HDPP), hydrogenated 4,4xe2x80x2-dihydroxydiphenyl, 1,4-cyclohexanedimethylol, 1,4-dihydroxycyclohexane, hydroxypivalylhydroxypivalate and 2-butyl-2-ethyl-1,3-propanediol or mixtures thereof; of which HDPP is particularly preferred.
Typical examples of a suitable compound C1 for the preparation of starting carboxy functional polyesters are aliphatic alcohols and cycloaliphatic alcohols, having primary or one secondary hydroxyl group and having from 1 to 6 carbon atoms such as neopentanol, 2-butanol, cyclohexanol, or a 1:1 adduct of a VERSATIC acid and a glycidylester of a VERSATIC acid, having from 5 to 13 carbon atoms.
Suitable compounds C2 are aliphatic and cyclo-aliphatic alcohols having one primary or one secondary hydroxyl group and having of from 1 to 6 carbon atoms and in addition one tertiary aliphatic carboxyl group, such as 1-methyl-4-hydroxycarboxylic acid, hydroxypivalic acid.
A typical example of a suitable compound D for use in the preparation of starting carboxy functional polyester is dimethylol propionic acid (DMPA).
A typical and preferred example of compound E, if any, is used for the preparation of the starting branched glycidyl functional polyesters, to be used for the process of the present invention, is trimethylol propane and a preferred example of compound E2 if any is used, is pentaerythritol.
It will be appreciated that the starting glycidyl functional polyester resins can be obtained by easy conversion of a precursor carboxy functional polyester resin with an excess epihalohydrin, in the presence of a suitable base and optionally a catalyst. Most conveniently epichlorohydrin is used.
It was found that those polyglycidyl ester resins to be used as starting material (II) for the process of the present invention, are preferred, which have been derived from carboxyl functional polyester resins (III), wherein Y ranges from 1 to 4, X simultaneously ranges from 1 to 6, Z ranges from 0 to 1 and Q ranges from 0 to 1, can provide the more preferred outdoor durable powder coating compositions. Most preferably polyglycidyl ester resins are used, wherein X ranges from 1 to 4, Y ranges from 1 to 2, Z=0, Q=0.
As indicated hereinbefore, the starting polyglycidyl ester resin can be reacted either with a carboxy functional polyester resins derived from the hereinbefore specified components (a) and (b) or with a component (a) alone.
A preferred coreactant for the polyglycidyl ester resin is formed by adducts of hexahydrophthalic anhydride (HHPA) and hydrogenated diphenylol propane (HDPP), adducts of 1,4-cyclohexanedicarboxylic acid(1,4-CHDA) and hydrogenated bisphenol A, in a molar ratio of from 4:1 to 1:1 and more preferably about 2:1, or HHPA or 1,4-CHDA alone or mixtures thereof.
More preferably adducts of 1,4-cyclohexanedicarboxylic acid and hydrogenated diphenylol propane (HDPP) in a molar ratio of 2:1, are used.
Examples of suitable catalyst which can be used for the process of the present invention may be selected from tetra(hydrocarbyl)-ammonium or -phosphonium compounds or metal salts/compounds such as stannous II octoate, basic compounds such as imidazoles and tertiary amines such as diazabicyclo undecene.
More preferred catalysts are ethyltriphenyl-phosphonium halide and more preferably ethyltriphenyl phosphonium iodide, tetra(alkyl)ammonium chloride, tetra(alkyl)ammonium iodide, and more preferably tetramethylammonium iodide, dimethyl dibenzyl ammonium iodide, diethyl dibenzyl ammonium iodide.
It will be appreciated that another aspect of the present invention is formed by the specific, advanced polyglycidyl esters obtainable by the hereinbefore specified process.
Due to the specific polymeric nature of these advanced polyglycidyl ester resin of the present invention, a relatively low level of toxicity can now be combined with excellent coating properties and attractive storage stability due to the increased Tg, whereas the melt viscosity is sufficiently low to enable a good processing of the resin into the final powder coating composition.
The curable outdoor durable powder coating compositions, forming another aspect of the present invention, may be prepared by addition of a cross-linking resin to the hereinbefore specified advanced polyglycidyl ester resin (I).
As cross-linking resin a precursor carboxy functional polyester resin (III) from which the intermediate initially prepared polyglycidyl ester (II) can be derived by glycidation, can be used. Preferably said precursor has a polyester chain microstructure which is rather similar to that of the final polyglycidylester.
The amount of cross-linking compound used in the powder coating compositions of the invention will normally be such so as to provide about equal amounts of reactive groups of the cross-linking compound and the epoxy groups present in the advanced polyglycidyl ester resin.
However, other suitable cross-linking resins can also be used in combination with the advanced polyglycidyl ester resins of the present invention, such as solid polyacids such as sebacic acid, 1,12-dodecanedioic acid; anhydrides such as polyazeleic polyanhydride; acid functional polyesters such as the reaction product of one mole of trimethylolpropane and 3 moles of hexahydrophthalic anhydride, the reaction product of 1,6-hexanediol with a molar excess of 1,12-dodecanedioic acid, the reaction product of 4 moles 1,10-decanedicarboxylic acid, 1.49 mols hexanediol, 0.47 mols 1,1,1-tris-(hydroxymethyl)-propane and 0.27 mols pentaerythritol, the reaction product of 4 mols 1,10-decanedicarboxylic acid, 1.2 mols hexanediol, 0.45 mols trimethylolpropane, 0.29 mols pentaerythritol and 0.21 mols dimethylolpropionic acid and the reaction product of one mole of hexa-methoxymethylmelamine and 3 moles of hydroxypivalic acid and amine-type curing agents.
The powder coating composition of the present invention may further comprise a catalyst and optionally other additives, as known in the art to be suitable for use in powder coating compositions.
Suitable catalysts are for example quaternary ammonium and phosphonium salts; metal salts/compounds such as for example stannous(II)octoate; basic compounds such as for example the imidazoles; and tertiary amines such as for example diazabicycloundecene.
The amount of catalyst used will usually be somewhere in the range of from 0.01 to 2% by weight based on the weight of the total powder coating composition.
Suitable cure times and cure temperatures of the powder coating compositions of the invention are those conventionally applied in connection with powder coating systems.